Usually a brushless motor is driven by appropriately exciting the motor's phase windings synchronously with the instantaneous position of the rotor. More precisely, this is done by sequentially exciting two of the motor's windings with positive and negative voltages respectively, leaving a third phase winding in a high impedance state. When driving a "sensorless" brushless motor, the non-excited phase winding is exploited for detecting the rotor's position.
According to a driving technique described and illustrated in European patent application No. 96830180.4, filed on Mar. 29, 1996, and assigned to the present assignee, on the motor's phase windings are sequentially applied or forced excitation voltages or currents. For these currents or voltages, rather than having a certain substantially constant level during each switching phase, have a certain profile (that is they have a non-constant value) which may be predefined, digitized and stored in a read only memory, such as, for instance, an EPROM or EEPROM.
The predefined cyclic or alternating profile may be sinusoidal or, more generally, the result of a Fourier sum of harmonics of a fundamental frequency. In practice, this innovative driving technique permits optimizing the torque characteristics of the motor by allowing for the predefinition of the most suitable cyclic waveform of the excitation voltage or current that is applied to or forced on the motor's phase windings. During each switching phase, the driving current or voltage does not correspond to a certain constant value as was common in more traditional driving systems. Instead it corresponds to a certain profile congruent with a portion of the whole predefined cyclic or alternate waveform. In other words, during each switching phase, the winding's driving signal level precisely follows a segment of the predefined waveform.
A second European patent application No. 96830190.3, filed on Apr. 4, 1996, also assigned to the present assignee, describes a highly efficient technique for synchronizing the sequence of excitation phases with respect to the actual rotor's position to optimize the torque characteristic of a driving system as that of the prior patent application No. 96830180.4. According to patent application No. 96830295.0, at each instant the three BJT or MOS half bridges, coupled to the respective motor's three phase windings, are driven in a PWM mode. Moreover, if the motor is connected in star configuration, with CT the star center and Vcc the supply voltage, then CT assumes a voltage equal to Vcc/2. Therefore, the maximum voltage swing that is available on each motor's winding is limited to .+-.Vcc/2. At the same time, the differential voltage swing (phase-phase) is limited to ##EQU1##